Frequency responsive circuits



R. C. SANDERS, JR

FREQUENCY RESPONSIVE CIRCUITS A ril 20, 1948.

Original Filed Nov. 50, 1942 Snv'eutor Gtforncg Patented Apr. 20, 1948 FREQUENCY 'RESPONSIVE CIRCUITS Royden C. Sanders, Jr., Hightstown, N. J., assignor to Radio Corporation-of America, a corporation. of Delaware Original application November 30, 1942, ScrialNo. 467,445. Divided and this application March 2, 1944, Serial No. 524,797

'2'Claims. 1

This application is a division of my copending United States application, Serial No. 467,445, filed November 30, 1942, now Patent No. 2,403,615, issued July 9, 1946, entitled Frequency responsive circuits, and assigned to the same assignee as the instant application.

This invention relates to frequency responsive circuits, .and more .particularly to a system for operating a relatively rugged relay in response to signals of predetermined or selected frequencies. When the input is above a preselected frequency, the relay is operated to one position; when the input is below this frequency the relay is operated to another position. Thus the device mayv be .used to control motors or indicators in response to changes in the frequency of an alternating current source of relatively low power capability.

One application of this system is in the control of aircraft by means of a radio altimeter of the type described on pages 86-90 of Civil Aeronautics Bulletin No. 29, published by the Civil A'ieronautics Administration. The radio altimeter providesan audio. frequency output directly proportional in frequency to the height of the airplane above the ground. The airplane maybe controlled to fiy at some selected .level by operating motors connected to the elevator surfaces incresponse to variations .in the output frequency of the altimeter from the frequency correspond ing to the selected level. A-lfrequency responsive relay system for this purpose must withstand the vibration and acceleration forces occurring in flight failure or false operation.

Accordingly, it is one object of this invention to provide an improved method of and means for controlling a relatively heavy and insensitive relay in response to variations from a predetermined value in the frequency of an alternating voltage. 7

Another object is to provide an improveddevice which is uniformly sensitive to frequency variations, independent of the selected frequency.

A further object is to provide an improved method of and means for controlling the damp ing or. a relay or other electromagnetic device. to prevent erratic operation in response to sudden variationsof the control voltageaor received signal.

These andother objects will become apparent to'those skilled in 'the'art upon consideration-of the following description, with reference to the accompanying drawing, which is aschematic circuit diagram of an embodiment of the invention.

A pentode amplifier tube is provided, with a grid leak 3, a self bias resistor 5, and a load resistor 7. The plate :of the tube 1 is coupled through a capacitor 9 to a double diode H. The plate l3 ofone diode section and the-cathodeilfi of the other diode section are connected to the capacitor'll. The cathode ll of the first section is grounded, and the .anode H! of the other section is connected to ground through a. capacitor 21. Thegrid23 .of an amplifier tube 25 is :connected to the diode plate l9:.th-rough a resistor 2 A load resistor 29' .is connected from :the anode 19 to the armfit3i of a tap switch 33. A plurality of resistors 35-are connected in series with an adjustable resistor :31 acrossa direct current :source38. The taps 39 of 'theswitch 33 are connected between the resistors 35 so that an adjustable portion of the direct current voltage may be applied to the load resistor 29. A relay.:4l is connected in the anode circuit of the tube 25.

The device operatesas follows:

Audio frequency voltage applied aoross the resistorsssiis amplified in :the tube I. The amplitude of the input "voltage is made high enough to cut off the tube on the negative peaks and cause saturation on the positive peaks. This clips the peaks ofthe amplified voltage, producing an output of rectangular wave form. The time constant of the condenser-9 and the resistor 1 is made sosmall that thecondenser il will be almostlOO percent disc'harged or charged inone half cycle of the highest frequency at which this device is to operate.

The positive half cycle of the rectangular wave causes the condensenilto be charged through the diode section l3, l1. Ihe negative half cycle of the rectangular wave discharges the condenser 9 through the diode section 15, I9 into the condenser '21 and hence; stores its discharge current in the "condenser 21. The con-denser '2idischarges "through the loadresistor 29, potentiometer 8-3 and the direct current source 38. The condenser 9 charges and discharges through a voltage range Esw equal .to the peak to peak amplitude of the rectangular wave voltage across the resistor '1 minus any average negative voltage .Ef developed across the condenser 2 l. Hence every cycle (Esw-l-EfiC' coulombs .or (Esw-l-Ef) (C) (F) amperes will be passed by the diode section #5, 19, where Fis the frequency of the applied signal, and C is the capacitance of the capacitor 9. If Eb is the positive bias potential developed between point 31 and ground, and R' is the resistance of the resistor 29, then Ef=Eb-(Esw |-Ef) (C') (F) (R) when E} is negative only. It is seen, therefore,

diodes l1, I3 and l5, [9 to maintain Ej at approximately zero volts. Thus B will not vary appreciable with frequency until the current (Esw-l-Ef) (C) (F) is large enough to cause E to become negative. Above that point Ef will vary with frequency becoming more nonlinear as Ef becomes an appreciable part of Esw. The rate of change of Ef with respect to frequency is:

(E (Esw+ Ef) (CR) dF l-CFR The condenser 2i discharges through the load resistor 29, potentiometer 33 and the direct current source 38.

Thus'the negative voltage Ef increases with frequency, becoming nonlinear with respect to frequency when E is an appreciable fraction of the square wave voltage Esw. The voltage at the Eb tap 3| is adjusted to a value corresponding to the frequency at which the relay is to be operated. When the frequency isbelow the selected value, the grid 23 of the tube 25 is positive and the relay is energized, maintaining the contacts 43 in their upper positions. When the voltage Ef becomes slightly greater in magnitude than the voltage at the tap 3|, the grid 23 becomes negative and the anode current of the tube: 25 decreases, reducing the energization of the relay 4| and causing the movable contacts 43 to go to their lower positions.

In order to vary the frequency at which the relay 4| is operated the tap 3| may be moved to the corresponding contact 39. The system is then operated as described above, cutting off the tube 25 as the frequency increases about the selected value. Thus the same number of cycles per second deviation will actuate the relay at any selected frequency. The resistor 21 and the capacitor 28 are so proportioned as to have a time constant equal to or greater than that of the capacitor 2| and resistor29. This causes the anode current of the tube25 to change at the same rate with a decrease in frequency as it does with an increase in frequency, preventing erratic operation of the relay M.

The invention has been described as a frequency responsive circuit for operating a relay in response to input of a predetermined frequency, a unidirectional voltage, related in magnitude to the frequency of the input, is derived. An adjustable unidirectional voltage is opposed to the frequency derivedvoltage, and the resulting difference voltage is used to control an amplifier tube connected to the relay.

I claim as my invention: I

1. A system for providing substantially uniform response to deviation in frequency of an input signal above and below a selected frequency, including a counter circuit including a capacitor and means for applying said input signal thereto, saidmeans including two parallel paths each including a respective rectifier, said rectifiers beingconductive in respectively opposite directions, one of said paths including a load resistor and a source of bias voltage predetermined in accordance with said selected frequency, connected in series with each other and said rectifier, and a storage capacitor connected across said load resistor and bias source, an amplifier tube including a control grid and a cathode, and means for applying the voltage across said storage capacitor to said control grid, said last mentioned means including a resistor connected between said load resistor and said grid and a capacitor connected between said grid and said cathode, the product of the values of said last mentioned resistor and said last mentioned capacitor being at least asmuch as the product of the values of said load resistor and said storage capacitor.

2. A system for providing substantially uniform response to deviation in frequency of an input signal above and below a selected frequency, including means, for limiting the amplitude of said signal to a predetermined value, a counter circuit including a capacitor and means for applying the output of said limiter means thereto. said means including two parallel paths, one comprising a rectifier and the other comprising a second rectifier connected to conduct in the direction opposite to said first rectifier, a load resistor and a source of adjustable bias connected in series with said second rectifier, and a storage capacitor connected across said load resistor and bias source, whereby charging and discharging of said first mentioned capacitor through said second path produces a voltage drop in said load resistor which is a non-linear function of the frequency of said input signal such that a given number of cycles per second change in said frequency causes a predetermined change in said voltage, drop substantially independently of the actualfrequency of said signal, an amplifier tube including a control grid and a .cathode, and means for applying the sum of the voltage drop across said load resistor and the voltage of said bias source to said control grid, said last mentioned means including 'a resistor connected between said load resistor and said grid and a capacitor connected between said grid and said cathode, the product of the values of said last mentioned resistor and said last mentioned capacitor being at least as much as the product of the values of said load resistor and said storage capacitor.

ROYDEN C. SANDERS, JR,

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 634,937 Buck Oct. 1'7, 1899 2,010,253 Barton Aug. 6,1935 2,139,489 Cockrell Dec. 6,1938 2,218,642 Hathaway Oct. 22, 1940 2,258,943 Bedford Oct. 14,1941 2,259,070 Krochmann Oct. 14, 1941 2,260,933 Cooper Oct. 28,1941 2,262,407 Rath NOV. 11, 1941 2,307,316 Wolff Jan. 5, 1943 2,362,503 Scott Nov. 14, 1944 

